1. Field of the Invention
The present invention relates to voltage control in a power circuit, especially to voltage control in a power circuit for supplying electric power to an electronic machine by utilizing voltages from conventional power sources for generating low and high voltages.
2. Description of the Related Art
Some electronic machines can use a plurality of power sources. In such electric machines, it is necessary to change various voltages to a constant voltage, and to use the constant voltage. Therefore, in such electronic machines, step-down power circuits for stepping down the voltage from power sources, and step-up power circuits for stepping up the voltage from power sources, are used together.
A conventional power circuit that applies a constant voltage, generally includes both a step-up circuit for stepping up a voltage from a power source, being lower than an output voltage, and a step-down circuit for stepping down a voltage from another power source, being higher than an output voltage. In such power circuits, energy loss tends to be large mainly because of heat loss caused by a series regulators used for stepping down voltage in a step-down circuit.
On the other hand, when using only one of a step-up and step-down power circuit, some problems regarding power supply may occur. For example, in a case where only a step-down power circuit and a battery that generates an input voltage higher than an output voltage are used, when the input voltage becomes lower than the output voltage, as a result of the input voltage dropping due to long time usage of the battery, an electric machine having the step-down power circuit may stop suddenly. This is because the input voltage can not be recovered to an acceptable level, due to the absence of a step-up circuit.